1. Field of the Invention
The present invention relates to a method for eliminating the partial, pulsatory discharges of the corona effect along the metallic conductors of an aerial, high voltage electric power transmission line when these conductors are wet.
The invention also relates to a conductor for an aerial, high voltage power transmission line, which allows the above-mentioned method to be carried out.
2. Description of Related Art
It is well known that a high voltage conductor passing through a gas, strongly ionizes the gas and generates a plurality of phenomenae known as "corona effects".
The generation of the corona effect along a high voltage conductor is explained by the fact that the electrode under voltage generates an electrical field in close proximity to its surface and that an aerial conductor acts with the ground as an electrode and counter-electrode, respectively.
As soon as an aerial conductor is under voltage, it generates an electrical field around its surface. If the so generated electrical field is greater than a given value corresponding to the critical limit of disruption of the air, then local ionization is produced all around the conductor. The more the surface of the conductor is curved, the greater is the ionization. Indeed, it is well known that the value of the electrical field generated by an electrode is a function of the curvature of the surface of the electrode.
When the conductor is maintained under a moderated voltage, there is no ionization of this type. However, if the voltage under which the conductor is maintained increases, the corona effect appears, usually in the form of a multitude of partial, electrical discharges appearing along the whole length of the conductor.
This disruption of the air due to ionization is accompanied in practice with a substantial loss of energy that may take the form of luminescent phenomena (glow-discharge), loss in power electromagnetic interference and/or audible sound.
The luminescent phenomena is due to the emission of photons by the atoms of the air when these atoms are excited by the electrons propulsed by the strong electrical field. The loss in power is due to the dissipation of the energy necessary for the ionization of the atoms, the neutralization of the ions at the surface of the conductor and more particularly, the alternating movement of the charges created in the air in the proximity of the conductor when the current is alternating. The electromagnetic interference is due to the impulses of current circulating along the conductor, which current generates Hertzian waves. Last of all, the audible sound is due to the impulsion phenomena of heating and dilation of the air which causes sonorous waves with a straight front and whose spectrum extends up to 20,000 hertz, that is in the field of the audible frequencies. The sound is also due to the oscillatory movement of the charges in the space in the neighborhood of the conductor when this conductor is maintained under alternating voltage, this phenomena also causing sonorous waves whose frequency is twice the frequency of the voltage, this is 120 hertz.
When use is made of an electrode having a very high surface curvature, the loss of energy by ionization of the air will usually be stationary and calm and will result in a luminescent phenomena, a continuous power loss and a sound whose frequencies are low (centered about 120 hertz) when the current is alternating. This kind of corona effect will be referred to hereinafter as "permanent discharges".
In the other case, for example, when the surface of the conductor has a low curvature, the loss of energy will be sporadic, turbulent and pulsatory and will result in a multitude of pulsatory discharges having a length in the range of about one centimeter and a duration of about 200 nanosecond. A loss in power very important radioelectric perturbations and an audible sound having harmonic component in the low frequencies (120 hertz) and another component whose frequency extends up to the limit of audibility, are produced because of the impulsions.
If, in practice, the above mentioned phenomenae all are very annoying from an economical point of view in the exploitation of the transmission lines because of the power loss they generate, they are more particularly annoying from an ecological point of view and are very detrimental for the persons who live in the neighborhood of the high (or very high) voltage transmission lines. The two particular phenomenae are, on one hand, the electromagnetic perturbations that produce interference in the reception of the modulated Hertzian waves (radio or television) and, on the other hand, the audible sound that is or can be very annoying. As a result, the implantation of a high voltage transmission line in a very specific region, always raises a plurality of very serious problems for the engineers who design the project, in particular when the line is close to crowded areas.
In order to solve the ecological problem, several studies have been carried out in some countries in an attempt to better understand the nature of the corona effect and maintain the various perturbations due to corona effect at the lowest possible level by acting on the factors of influence of the corona effect that are presently known.
These studies have shown that it is possible to reduce the drawbacks associated with the corona effect or their consequences on a given transmission line, by
(a) making a judicious selection among the geometrical characteristics of the line for reducing the value of the electrical field at the surface of the conductor; and/or
(b) using a bundle of cohductors for artificially reducing the electrical field at the surface of the conductors; and/or
(c) avoiding scratching the surface of the conductors during their installation as the very high curvature of each irregularity with respect to the average curvature of the conductor surface creates a punctual zone of strong ionization on the surface of the conductor and thus lowers in a substantial manner the limit at which the corona effect appears.
With a judicious selection of the geometrical characteristics of the line such as the diameter and space arrangement of the conductors, it is almost possible to completely eliminate the drawbacks due to the corona effect, as can be easily noted in practice on most of the existing high voltage transmission lines which, when the weather is fine and the atmosphere is not too much polluted, actually do not generate any perturbations directly attributable to the corona effect.
The second solution proposed hereinabove is also interesting but it involves installation costs that are very high.
If all the above-mentioned solutions have some advantages, they also have a common drawback. This common drawback is that none of these proposed solutions actually controls the main source of iongenerating irregularities, namely the deposit of drops of water onto the surface of the conductors by the rain, drizzle, mist, dew or melting snow. Each drop of water formed on the conductor constitutes an irregularity of the surface of the conductor which substantially increases the local ionization and substantially lowers the limit of the appearance of the corona effect.
This negative effect of the rain is well known by those in charge of the high voltage transmission lines, who know that the corona effect and the electrical loss caused by this effect are almost negligible when the weather is fine but occurs as soon as it rains or the lines are wet.
In this regard, the above-mentioned studies have shown that when it rains, each drop of water falling onto the conductor and adhering to the surface of this conductor, is deformed under the effect of the electrical field and transformed into a small "tip" whose sharp end creates a very important, local ionization. This liquid "tip" is known to generate much more partial, pulsatory discharges than permanent discharges.
As has been previously explained, the corona effect generated by the rain is very annoying for those responsible for the transmission lines because of the electrical loss it causes. This corona effect is also very annoying from an ecological point of view when it takes place in the form of pulsatory discharges instead of permanent discharges, as the pulsatory discharges involve much more acoustical and electromagnetic perturbations than the others.
To solve this drawback directly associated with the rain, several solutions have already been proposed. One of these solutions was experimented with in Belgium by an engineer, Mr. Lecat. It consists of fixing a plurality of small metallic tacks at regular intervals along the conductor. Each of these tacks generates a power glow-discharge at its ends and prevents the generation of partial pulsatory discharges that are the source of electrical and acoustical perturbations when it rains. Although this solution is satisfactory from an ecological point of view, it has however a very important drawback for those in charge of the electrical transmission lines. Indeed, this solution involves the permanent creation of luminescent discharges along the conductor, which, in practice, is not necessary when the conductor is dry and which results in very important power loss along the lines.
Another solution proposed for overcoming the problem due to the rain, consists of making each conductor hydrophobic to ensure a better ejection of the water falling onto its surface. Such a conductor is, however, very difficult to manufacture in practice.
A further proposed solution consists of making each conductor hydrophilic to ensure a better circulation and repartition of the rain onto its surface. This solution has proved to be positive but is not sufficient in practice.
To overcome the problem due to the rain, it has also been proposed to spiral a very thin metallic wire around a bundle of separate conductors. This spiralled wire forms an electrode having a very strong curvature, which extends along the whole length of the bundle and generates a permanent and stable corona effect resulting in permanent discharges not annoying from an ecological point of view. These permanent discharges usually eliminate the production of pulsatory discharges on the bundles even when it rains. This solution, as the first solution described hereinabove, has the important drawback of generating a corona effect in a permanent manner, along the whole length of the conductor, such being in practice not necessary when the conductor is dry and moreover, generating a substantial excess in power loss.